int value under 10 convert to string two digit number

The accepted answer is good and fast:

i.ToString("00")

or

i.ToString("000")

If you need more complexity, String.Format is worth a try:

var str1 = "";
var str2 = "";
for (int i = 1; i < 100; i++)
{
    str1 = String.Format("{0:00}", i);
    str2 = String.Format("{0:000}", i);
}

For the i = 10 case:

str1: "10"
str2: "010"

I use this, for example, to clear the text on particular Label Controls on my form by name:

private void EmptyLabelArray()
{
    var fmt = "Label_Row{0:00}_Col{0:00}";
    for (var rowIndex = 0; rowIndex < 100; rowIndex++)
    {
        for (var colIndex = 0; colIndex < 100; colIndex++)
        {
            var lblName = String.Format(fmt, rowIndex, colIndex);
            foreach (var ctrl in this.Controls)
            {
                var lbl = ctrl as Label;
                if ((lbl != null) && (lbl.Name == lblName))
                {
                    lbl.Text = null;
                }
            }
        }
    }
}

I can't believe nobody suggested this:

int i = 9;
i.ToString("D2"); // Will give you the string "09"

or

i.ToString("D8"); // Will give you the string "00000009"

If you want hexadecimal:

byte b = 255;
b.ToString("X2"); // Will give you the string "FF"

You can even use just "C" to display as currency if you locale currency symbol. See here: https://docs.microsoft.com/en-us/dotnet/api/system.int32.tostring?view=netframework-4.7.2#System_Int32_ToString_System_String_

i.ToString("00")

or

i.ToString("000")

depending on what you want

Look at the MSDN article on custom numeric format strings for more options: http://msdn.microsoft.com/en-us/library/0c899ak8(VS.71).aspx